Abstract

Ediacaran (ca. 635–541 Ma) marine carbonates capture a global
δ13C carbon isotope excursion to extremely negative values (~–12‰)—known
as the Shuram excursion (SE)—that cannot be explained by
conventional mass balance scenarios. Furthermore, the carbon isotopic
variation of bulk organic matter (OM) does not mirror that of
carbonate through the excursion, suggesting that the OM reflects a
mixture of different sources. To evaluate this hypothesis, we investigated
thermally immature marine sedimentary rocks that record
the SE from the Sultanate of Oman. Compound-specific carbon
isotopic analyses of the extractable hydrocarbons reveal low δ13C
values of long-chain (>C20) n-alkanes and mid-chain monomethyl
alkanes as low as –40‰. Such light signatures are rare in marine
rocks of any age and provide evidence that the SE reflects a primary
carbon cycle perturbation. The magnitude of the SE recorded
in these organic phases is smaller than observed in carbonate and
implies that the primary perturbation to dissolved inorganic carbon
(DIC) was at least 5‰–7‰, and more likely 7‰–12‰, in magnitude
when correcting for end-member source mixing. Due to isotopic
differences in stratigraphic patterns of the different organic
compounds, we propose that bulk organic carbon (both bitumen
and kerogen) reflects source mixing between two distinct pools that
previously masked the excursion in bulk δ13Corg measurements. OM
sources were derived both from autotrophs fixing 13C-depleted DIC
and from a less 13C-depleted heterotrophic microbial biomass feeding
on a marine OM pool sustained by petroleum expelled from
older sedimentary OM. Expulsion of these sedimentary fluids also
helps explain both the duration and magnitude of the SE.